<meta charset="utf-8">
(#) Kotlin nullability annotation

!!! ERROR: Kotlin nullability annotation
   This is an error.

Id
:   `KotlinNullnessAnnotation`
Summary
:   Kotlin nullability annotation
Severity
:   Error
Category
:   Correctness
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   7.3.0 (September 2022)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/KotlinNullnessAnnotationDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/KotlinNullnessAnnotationDetectorTest.kt)

In Kotlin, nullness is part of the type system; `s: String` is **never**
null and `s: String?` is sometimes null, whether or not you add in
additional annotations stating `@NonNull` or `@Nullable`. These are
likely copy/paste mistakes, and are misleading.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/test.kt:7:Warning: Do not use @NonNull in Kotlin; the
nullability is already implied by the Kotlin type String not ending with
? [KotlinNullnessAnnotation]
fun testWarning(@NonNull string: String) { }
                --------
src/test/pkg/test.kt:11:Error: Do not use @Nullable in Kotlin; the
nullability is determined by the Kotlin type String not ending with ?
which declares it not nullable, contradicting the annotation
[KotlinNullnessAnnotation]
fun testError(@Nullable string: String) { }
              ---------
src/test/pkg/test.kt:15:Error: Do not use @NonNull in Kotlin; the
nullability is determined by the Kotlin type Number? ending with ? which
declares it nullable, contradicting the annotation
[KotlinNullnessAnnotation]
fun testError(@NonNull number: Number?) { }
              --------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package test.pkg
import androidx.annotation.NonNull
import androidx.annotation.Nullable

// Here we have an already non-null string annotated with @NonNull;
// warn, since this is redundant
fun testWarning(@NonNull string: String) { }

// Here we have a non-null string which has been annotated with @Nullable,
// which is totally misleading; the annotation is wrong
fun testError(@Nullable string: String) { }

// Here we have a nullable string which has been annotated with @NonNull,
// which is totally misleading; the annotation is wrong.
fun testError(@NonNull number: Number?) { }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/KotlinNullnessAnnotationDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("KotlinNullnessAnnotation")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("KotlinNullnessAnnotation")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection KotlinNullnessAnnotation
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="KotlinNullnessAnnotation" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'KotlinNullnessAnnotation'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore KotlinNullnessAnnotation ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

<!-- Markdeep: --><style class="fallback">body{visibility:hidden;white-space:pre;font-family:monospace}</style><script src="markdeep.min.js" charset="utf-8"></script><script src="https://morgan3d.github.io/markdeep/latest/markdeep.min.js" charset="utf-8"></script><script>window.alreadyProcessedMarkdeep||(document.body.style.visibility="visible")</script>